Nuclear reactor component cladding material



Jan. 27, 1959 J. E- DRALEY ET AL NUCLEAR REACTOR COMPONENT CLADDING MATERIAL Filed Mar ch 2, 1956 2 Sheets-Sheet 1 I/I/E/G/ /T G/U/V 2 M/AL /6FAM6 1 58 CM I I .70 J5 INVENTORS a /waw J. E. DRALEY ET AL 7/M am 7557;

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NUCLEAR REACTOR COMPONENT CLADDING MATERIAL 8 f 3 w w i? A 2 WW, 5 W @M a if 7 m a? a L J a Jan. 27, 1959 Filed March 2, 1956 NUCLEAR REACTOR COMPONENT CLADDING MATERIAL Joseph E. Draley, ClarendonHills, and Westly E. Rather, Skokie, 111., assignors to the UnitedStates of America as represented by the United States Atomic Energy Commission Application MarchZ, 1-956, Serial No. 569,215

C 11 Claims. or. 204-1933 This invention relates to an improvement in nuclear reactors. In more detail the invention relates to an improvement in a subassembly for nuclear reactors comprising a moderator, coolant tubes contained in the moderator, and fuel elements contained within the coolant tubes wherein the coolant tubes and the casings or coatings for the fuel elements are formed of an alloy of aluminum and nickel which is more resistant to corrosion by water under radiative conditions than is aluminum itself-and may be used at temperatures much higher than aluminum alone can be used. A more limited aspect of the invention relates to a fuel element comprising a core of a fissionable material and a casing or coating of a corrosion resistant aluminum alloy.

Fermi et al.-disclose in-Figures 37 to 39 of Patent No. 2,708,656 a water-cooled reactor including a solid moderator, coolant tubes imbedded in the moderator and fuel elements contained within the coolant tubes. Aluminum is specified as being the preferred material for use in construction of the coolant tubes and the casings for the fuelelements. Commercial wrought aluminum containing .3 to .5% iron, known as 28 alloy may be used up to a water temperature of approximately 200 C. and is frequently used in nuclear reactors. However, at elevated temperatures most aluminum alloys in water suffer severe penetrating attack, resulting in relatively rapid destruction of the material. This occurs because a transition in corrosion behavior takes-place above approximately 200 C. Above that temperature the metal develops blisters of mixed aluminum oxide and metal at an accelerating rate, with penetration into the metal, and it is rapidly rendered useless. Accordinglycorm mercial aluminum and other commercially available aluminum alloys are not suitable for use in reactors which are operated under conditions where the cooling water may reach a temperature over 200 C. Use of aluminum in reactors which are designed .to. operate under 200 C. may be. dangerous since hot spots may develop in the reactor which could cause spot failure of the coolant channels or the fuel casings. Other materials of construction are not as suitable as aluminum either because of cost or because of unfavorable neutron capture cross section.

It has been found that an alloy of aluminum and nickel is notsubjectto intergranular corrosion bywater at-temperatures up to 350 C. andistherefore suitable for use in water-cooled reactors-in whichthewater may attain a temperature ofover 200 C. Examples of such' reactors include the water-cooled,graphite-moderated reactor disclosed on.pages 37-39 of thepreviously cited patentto Fermi .et .al. and the water-cooled, water-moderated reactor disclosed in application-Serial'No. 518,427, filed June 28, 1955,.in thename of Samuel Untermeyer. As the cross section of nickel-is. higher-than that of aluminum, it is desirable to employ an alloy of aluminum and nickel containing a minimum amount of nickel. It

2,?Ll76 Patented Jan. 2?, 1959 the alloy makes it possible to reduce the amount of nickel to a point where the total neutron capture cross section of the alloy is reduced while retaining the desired corrosion resistance. Therefore, for optimum results in a reactor a ternary alloy of aluminum, nickel and iron may be employed.

' it is accordinglyan object of this invention to pro-- vide a subassembly for a water cooled nuclear reactor in which the casings for the fuel elements and the coolant tubes are formed of a material which is resistant to corrosion by water at high temperatures.

It is a further object of the invention to provide a coating or casing for bodies of a fissionable material which is resistantto corrosion by water at temperatures above 200 C. so that the bodies may be used as fuel elements in a water-cooled neutronic reactor.

it is also an object of the invention to provide an aluminum-nickel alloy for use in a neutronic reactor which combines resistance to high temperature water corrosion with a'low neutron capture'cross section.

These and' other objects are accomplished by the use of an alloy of aluminum and nickel in those portions of a water-cooled 'neutronic reactor which come into contact with-the water. v

The invention will be further described by reference to the accompanying drawings wherein:

Figure 1 is a perspective view of a subassemblyfor a neutronicreactor comprising moderator, coolant tubes, and fuel elements in which the subassembly is shown in partial cross section;

Figure 2 is a perspective view of a different type of fuel element in which a part of the fuel element is broken away;

Figures 3 to 6 are graphs illustrating the improved results attainedby the use of this invention.

As shown in Figure '1 the subassembly comprises a solidmoderator 10, a coolant tube11,'and ribs 12 which space fuel elements 13 from the tube 11, forming a coolant channel 14 therebetween. Fuel elements -13-are composed of cylindrical uranium. slugs 15 provided with a protective jacket or casing 16. The arrangement of elements is that shown in Figure 39 of the Fermi et al. patent previously referred to. The coolant water passes through the channel 14. A water-cooled reactor may be operated under conditions whereby this water may reach a temperature above that at which commercial 28 alloy disintegrates in a short time. Therefore, the coolant tube-11 andthe protective jacket 16 in the subassembly shown in Figure l are formed of an alloy of aluminum and nickel.

.The fuel element may also be constructed so that cooling'fluid ilows both inside and outside of the uranium slug. In this case there is a channel forthe cooling fluid passing through the center of the slug and the hereinafter described alloy is used to coat both inside and outside of the uranium slug.

Figure 2 shows a different type of fuel element which maybe constructed with a casing of an aluminumnickel alloy. The fuel element'consists of .a core 20 which may be uranium and of a jacket designated by reference numeral 21. The core 20 is enclosed in a rectangular block 22 made of the aluminum-nickel alloy according to this invention and in two cover plates, 23) and 24, also made of the aluminum-nickel alloy. Ele merits 22, 23' and 24 form the jacket 21. The entire assembly has been subjected to rolling whereby bonding of the various elements has'been attained. A'fuel element of thistype may be contained within a coolant tube of the same or different shape to that shown in Figure 1. In any case the coolant tube may likewise be made of the same alloy.

' nace, using Alundurn crucibles.

The alloy employed consists of aluminum alloyed with a minimum of 2% nickel or in the alternative with a minimum ,of .5% nickel and .3% iron. alloy is most suitable for use in a reactor since the total neutron capture cross section is less than that of the binary alloy.

The alloy may be prepared from high purity aluminum or from 28 alloy. If the starting point is high purity aluminum, either 2% nickel may be added to it or .5 nickel and .3.% iron. If the starting material is 25 alloy, .5% nickel only need be added as commercially pure aluminum, or 28 alloy, always contains at least 3% iron.

The alloys were made in several different ways. Vacuum melting and casting were performed in a suitable furnace using pure graphite crucibles; A minimum vacuum of mm. of mercury was maintained. Air cast alloys were made in a small resistance-heated fur- An induction furnace (6 kw.) was used exclusively after it became available, since the'inductive stirring action rapidly dissolved the alloyin g elements.

In preparing the alloy the starting material, either high purity aluminum or commercially pure aluminum, was meltedas above described. Silicon was added to the melt as a 12% master alloy and iron was added as an 8.5% master alloy. Other elements Wereadded directly and stirred into the melt at temperatures up to 950 C.

The alloys were prepared and cast in sheets approximately one-half inch in thickness. The sheets were cold rolled to approximately one-eight inch in thickness as it was observed that some as cast commercially pure aluminum samples are less effected by high temperature water corrosion than formed aluminum shapes. This ability to withstand corrosion of as cast samples was destroyed by working of the casting.

Sample size and shape varied with the material available but in most cases the specimens were approximately three-quarters inch by three inches by three thirty-seconds inch. A three thirty-seconds inch hole was drilled near one end. A fresh surface was prepared by wet grinding to four hundred grit with abrasive paper on a metallographic polishing wheel. Samples were then measured, degreased in freshly distilled methyl alcohol. and weighed on an analytical balance.

Corrosion tests were performed in two kinds of apparatus. In the first or static apparatus, stainless steel ,autoclaves (175 milliliter capacity) were filled approxi- ,mately two-thirds full of the test solution. The solu- .tion wasdegassed by boiling in the autoclave just before arranged so that up to 30 gal. per minute of water can be pumped therethrough at the desired temperature. The pressure within the piping system was 1000 pounds per square inch.

Static tests have been carried out at 315 C. to determine the elfect of the addition of nickel on the corrosion of aluminum, by water at high temperatures. The first two tests in Table 1 are control tests on high purity aluminum and commercially pure aluminum. The other tests were carried out on samples of 2S alloy consisting of 99.2% aluminum, .4% iron and .4% of impurities (mainly silicon) to which had been added varying amounts of nickel.

The ternary 4 t. 1 Table 1 q [Weight gain, mgJcmfl] Exposure time, hours 1 Sample disintegrated within 24. hours.

2 Badly blistered-test discontinued.

These tests show that each of the samples containing nickel was vastly superior to the samples containing no nickel. Samples of high purity aluminum and of commercially pure aluminum completely disintegrate before the end of the test period, whereas samples containing nickel were still in good condition with moderate smooth deposits of corrosion product formed on them. It should be noted that iron was present in the samples containing nickel as the alloys were prepared from commercially pure aluminum which always contains a small amount of Iron.

Upper limits on the amount of nickel which may be included in the alloy have not been determined as an alloy which contains the minimum amount of nickel consistent withsatisfactory-corrosion resistance at the temperatures at which the reactor is to be operated is preferable. However, samplesincluding up to 4% nickel have been prepared and are satisfactory.

The results tabulated in Table 1 are plotted in Figure 3. The graphs showthat there is very little effect on corrosion rate caused by varying the nickel content of the alloy from .5% to 2.0% when other variables are held as constant as possible. Therefore, data from a number of tests on castings containing from about 5% to 2.0% nickel were plotted in Figure 4. The slopes of these curves give a rough estimate of the effect of temperature on corrosion rates. At 290 C. the corrosion rate turns out to be .0038 cm./yr. (1.8 mils/year),vat 315 C. the corrosion rate is .0085 cm./.yr. '(3 mils/year), and at 350 C. the corrosion rate is .024 cm./yr.'(9,mils/year).

Table 2 compares the corrosion of a sample of annealed 28 alloy and a sample of an annealed alloy containing 1% nickel in 25 alloy. The test was carried out at 315 C. It' is seen that the improvement in corrosion resistance is not destroyed by heat treatment of the sample.

' Table 2 28 alloy Disintegrated within four hours. 1% Ni in 28 alloy. Good condition after two weeks.

Table 3 Time,'hours: Weight gain, mg/crn. 20 .6 60 1.4

much worse at a temperature of 275? C. than it is at '315? C. for a sample containing nickel. Since corrosion increases with increased temperature as shown, for example in Figure 4, this illustrates the tremendous improvement effected in the alloy by the incorporation of a smallamount of nickel therein.

As iron and silicon are invariably present in commercial 28 alloy, tests were made to determine the eiiect of iron and silicon on the corrosion resistance of aluminum. In the first group of tests iron and nickel were added to a high purity aluminum master alloy. The temperature at which the tests were carried out was 350 C.

Table 4 Composition, percent Metal corroded, Time, mgJcm. days Ni Fe Si 1 Disintegrated.

These tests indicate that an alloy containing only .66% nickel must contain iron to be corrosion resistant at 350 C. and that silicon is not essential. Another group of tests was run to determine the amount of iron that is necessary and the results are given in Table 5.

Table 5 Composition,

percent Time,

- days Remarks Ni Fe Si 50 00 0. 1 Disintegratcd. 50 10 O1 1 D0. 50 22 01 1 Areas of surface blistering. 50 31 O2 1 Good condition.

.3% iron are equally corrosion resistant and that a sarn- L) ple of high purity aluminum containing 2% nickel, al-

though not necessarily the equal in corrosion resistance to the ternary alloys, shows great improvement over 28 alloy or high purity aluminum. Apparently therefore the amount of nickel required varies inversely as the amount of iron present up to a maximum amount of iron of about 3%. More iron can be present but does not permit of a reduction in the amount of nickel.

Both iron and silicon were added in another group of tests.

Table 6 Composition Metal corroded, Remarks Time, mgJcmfi days Ni Fe Si 50 30 30 5.1 Some blistering 6. 3 50 1. 11 33 3. 9 Good condition 6. 3

Silicon accordingly appears to be detrimental but can be accommodated by addition of sufiicient iron or nickel.

In order to simulate more closely the conditions in a reactor, a series of dynamic flow tests were carried out in thedynamic apparatus described above. These tests were carried out at 260 C. on an alloy of aluminum and nickel containin .9% Ni. The results obtained from these tests are plotted in Figures 5 and 6. The tests of Figure 5 were carriedout at a pH of 6.5 and of Figure 6 at a pH. of 5. For comparative purposes the results of static tests at the same temperature are also plotted in Figures 5 and 6. Each point that is plotted on the curves represents the average weight gain of a number of samples.

As expected, the amount of corrosion is greater in the dynamic tests than in the static tests and the amount of corrosion increases with increased flow rate of water past the sample. Likewise the corrosion rate increases with increased flow rate but the increase is slight. At the end of 60 days the samples were still in good condition. High purity aluminum and 28 alloy will completely disintegrate under the conditions of these tests before the expiration of the 60-day period.

Comparison between Figures 4 and 5 indicates that corrosion is somewhat less at lower pH. This indicates that acid should be added to the solution for optimum results. I

The above-described dynamic corrosion tests were all carried out by subjecting the samples to the corrosion effects of a stream of water Which Was circulated in a closed system and in which the system was not vented to the atmosphere. It has been found that much more corrosion is found in samples which were subjected to corrosion by a stream of water which was not recirculated or in which a recirculating system was vented to the atmosphere than is shown in the above-described tests. However, under the last mentioned conditions samples exposed'to radiation in a nuclear reactor are as resistant to corrosion as were the samples in the described corrosion tests.

Sample plates formed of an aluminum alloy containing small amounts of nickel and iron were placed in cartridges similar to those used to hold fuel elements for nuclear reactors. Such cartridges are conventionah ly formed of a number of thin plates held so that water or other coolant can flow through the cartridges around the plates. One of these cartridges was placed in a nuclear reactor and immersed in a stream of water. Another cartridge was immersed in the same stream of water outside the field of radiation of the reactor. The rate of how of the water was approximately 30 feet per second, The testing was intermittent and included hours at 260 C., hours at 238 C.243 C, 325 hours at 220227 (3., and 300 hours at 2057-!0" C. The plates that were within the reactor received an estimated thermal flux of exposure of 1 l0 neutrons per cm. In addition both cartridges were subjected to the corrosive effect of a stream of water having a temperature between 200 C. and 260" C. for 75 hours and at a temperature below C. for another 400 hours outside the field of radiation of the reactor.

The plates, as mounted in both cartridges, were arranged in two rows of three plates each with Row 1.2.1: the left end. Row 1 of the in-pile cartridge was closest to the reactor core and received the highest flux.

The iron present in the samples was that present in the 28 alloy from which the samples were prepared.

Thus we see that the corrosion increases with an increase in distance from the core and that the out'-of pile samples show a large increase in corrosion over inpile samples. sistance to corrosion and the alloy may therefore be used to form coolant channels and casings or coating for fuel elements in a nuclear reactor.

A working hypothesis has been developed to explain the corrosion of aluminum by water atshigh temperatures and the improvement in corrosion resistance ob tained by incorporation of nickel into an aluminumnickel alloy. It is well known that a surface layer of aluminum oxide forms immediately .on aluminum on exposure of the aluminum to water. that protons from the solution difiuse through the aluminum oxide barrier layer and are reduced to atomic hydrogen at the metal-oxide interface. The hydrogen atoms may combine to formrnolecules or diffuse into the metal. Apparently neither the atoms nor molecules can readily diffuse back out through the oxide film. It is assumedthat at elevated temperatures a significant fraction difiuses into the metal and collects at rifts or cavities to form noncliffusing molecular hydrogen. Pressure builds up within these rifts, cracking or bulging the soft metal into blisters. When these blisters rupture, water is admitted to the fresh metal producing more hydrogen at a point considerably below the normal metal surface. This process thus may become self-accelerating.

According to this theory an alloying agent which would provide a second phase of low hydrogen overvoltage to act as local corrosion cathodes for the rest of the aluminum grain would protect the alloy from this type of corrosion. The hydrogen would be released on the second phase rather than at the metal-oxide in terface.

In order to test the theory other metals other than nickel were added to aluminum and the resulting alloy was tested for corrosion resistance. The alloys tested consisted of 2% of the metal in high purity aluminum. Of those tested cobalt, iron, copper, and platinum gave fair results on corrosion test at 315 C. Each of these has a low hydrogen overvoltage. Cobalt and copper alloys were superior to the platinum and iron alloys but all were inferior to a vacuum-cast 1% nickel alloy.

Other elements tested included lead, tin, bismuth, and cadmium. Samples made from alloys containing 2% of these elements in high purity aluminum disintegrated I on testing at 315 C. This is as expected since each of these elements has a high hydrogen overvoltage.

Protection of aluminum against disintegration by water at high temperatures may also be obtained by adding small quantities of certain salts to the water and maintaining a low pH. 50 parts per million NiSO was added to the water used in a static corrosion test, and the water was acidified to a pH of 4 with sulfuric acid. After 48 hours the sample showed no blisters wherea a control sample was badly blistered. Other salts such as The in-pile samples show satisfactory re- It is postulated 2. A subassembly for a water-cooled nuclear reactor comprising coolant tubes formed from and fuel elements clad in a ternary alloy of aluminum which contains from .5 to 2% nickel and approximately .3% iron;

3. A subassembly for a water-cooled nuclear reactor comprising a solid graphite moderator, coolant tubes imbedded in the moderator, and fuel elements disposed within the coolanttubes wherein the coolant tubes are formed from and the fuel elements are clad in a ternary alloy of aluminum containing approximately .5 nickel and .3% iron.

4. A fuel element for nuclear reactors having a high corrosion resistance to water at high temperatures consisting of a core of a fissionable material and a jacket all around the core, said jacket consisting of a binary alloy of aluminum which contains between 2% and 4% nickel. i

5. A fuel element for nuclear reactors having a high.

corrosion resistance to water at high temperatures consisting of a core of a fissionable material and a jacket all around thecore, said jacket consisting of a ternary alloy of aluminum which contains approximately .5% nickel and .3% iron.

6. In a water-cooled graphite-moderated nuclear reactor the improvement comprising coolant tubes imbedded in the graphite moderator and fuel elements containing a fissionable material disposed in the coolant tubes wherein said coolant tubes are formed from and said fuel elements are clad in an alloy consisting of 0.5 to 4.0% nickel, from .3 to 0% Fe, the balance being aluminum, the relative amount of iron being high when the amount of nickel is low.

Co++, Cd++, Sn' Cu+ and Pb++ may also be used.

Other nickel salts may also be used and the amount of Ni++ may be reduced as low at 5 p. p, m. Comparative tests without pH control indicated that acid conditions are essential to satisfactory results.

It will be understood that this invention is not to be limited to the details given here nor to the theory promulgated here but that it may be modified within the scope of the appended claims.

What is claimed is:

1. In a water-cooled nuclear reactor, the improvement comprising coolant tubes formed from and fuel elements clad in a binary alloy of aluminum which contains between 2% and 4% nickel.

7. A fuel element for nuclear reactors having a high corrosion resistance to water at high temperatures consisting of a core of fissionable material and a jacket all around the core, said jacket consisting of an alloy consisting of 0.5 to 4.0% nickel, from .3 to 0% Fe, the balance being aluminum, the relative amount of iron being high when the amount of nickel is low.

8. In a water-cooled graphite-moderated nuclear reactor the improvement comprising coolant tubes imbedded in the graphite moderator and fuel elements containing a fissionable material disposed in the coolant tubes wherein said coolant tubes are formed from and said fuel elements are clad in an aluminum alloy selected from the group consisting of binary alloys containing 2 to 4% nickel and ternary alloys containing 0.3 to 0.5% iron and 0.5 to 2% nickel.

9. A fuel element for nuclear reactors having a high corrosion resistance to water at high temperatures consisting of a core of fissionable material and a jacket all around the core, said jacket consisting of an aluminum alloy selected from the group consisting of binary alloys containing 2 to 4% nickel and ternary alloys containing 0.3 to 0.5% iron and 0.5 to 2% nickel.

10. In a water-cooled graphite-moderated nuclear reactor the improvement comprising coolant tubes imbedded in the graphite moderator and fuel elements containing a fissionable material disposed in the coolant tubes wherein said coolant tubes are formed from and said fuel elements are clad in an alloy containing 0.5 to 4% nickel and an amount of iron up to 0.3%, the balance being aluminum. 7 i

11. A fuel element for nuclear reactors having a high corrosion resistance to water at high temperatures consisting of a core of fissionable material and a jacket all around the core, said jacket consisting of an alloy containing 0.5 to 4% nickel and an amount of iron up to 0.3%, the balance being aluminum.

(Other references on following page) OTHER REFERENCES The Aluminum Data Book, Reynolds Metal Co., 1950, Louisville, Ky., pp. 48, 49, 53..

Proceedings of the International Conference on the Peaceful Uses of Atomic Energy, vol. 9, United Nations, N. Y. (1956), held in Geneva Aug. 8-20, 1955, pp. 398, 400, 401.

TID-7506, Pt. I, U. S. AEC, Papers presented at the Technical Briefing Session held at Idaho Falls, Idaho,

'10 Nov. 1-2, 1955, pp. 5-16. (Available AEC Technical Inf. Ser., Oak Ridge, Tenn.).

T'he Reactor Handbook, vol. 3, AECD-3647, publ. by Technical Information Service, U. S. Atomic Energy Comm., declassified edition, February 1955, pp. 9, 10, 33, 35.

U. S. Atomic Energy Comm., H W-37636, Jan. 11, 1956,1115. 9-10.

V UNITED STATES PATENT OFFICE r CERTIFICATE 0F CGRRECTION 3 Patent No. 2,871,176

January 27, 1959 Joseph E. Draley et al.

It is hereby certified that error appears in the-printed specification of the above numb ered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, Table 1, first column,

fifth line thereof, for "5% nickel" read a5% nickel line 44, for "L8" read 1,5

\ Signed and sealed this 1st day of March 1960a (SEAL) Attest:

"{ KARL HQ AXLINE I ROBERT C. W ATSQ Attesting Oflicer Commissioner of Pat:

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,871,176 January 27, 1959 Joseph E. Draley et al.

It is herebj certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, Table 1, first column, fifth line thereof, for "5% nickel" read ,5% nickel line 44, for "1,8" read 1.5

Signed and sealed this 1st day of March 1960.

(SEAL) Attest:

KARL. H, AXLINE ROBERT C. WATSO Attesting Oificer Commissioner of Pat 

8. IN A WATER-COOLED GRAPHITE-MODERATED NUCLEAR REACTOR THE IMPROVEMENT COMPRISING COOLANT TUBES IMBEDDED IN THE GRAPHITE MODERATOR AND FUEL ELEMENTS CONTAINING A FISSIONABLE MATERIAL DISPOSED IN THE COOLANT TUBES WHEREIN SAID COOLANT TUBES ARE FORMED FROM AND SAID FUEL ELEMENTS ARE CLAD IN AN ALUMINUM ALLOY SELECTED FROM THE GROUP CONSISTING OF BINARY ALLOYS CONTAINING 2 TO 4% NICKEL AND TERNARY ALLOYS CONTAINING 0.3 TO 0.5% IRON AND 0.5 TO 2% NICKEL. 